Zirconium alloys



' f it I States 3,972,478 ?atented Jan. 8, 1263 tic This invention relates to zirconium alloys suitable, inter alia, for use as structural materials for the fuel element assemblies of nuclear reactors.

Among the properties required for a reactor structural material are:

(i) Low neutron absorption cross section; (ii) High creep strength together with adequate ductility;

and when a C cooled reactor is in question; (iii) Corrosion resistance in carbon dioxide.

It is known that dilute zirconium-copper alloys, i.e. alloysof Zr with a small percent of Cu, have a high resistance to corrosion at elevated temperatures in carbon dioxide atmospheres. Unfortunately the creep strength of these binary alloys is unacceptably low and a further alloying addition is necessary to improve this property. Any such addition must, however, be made with due regard to the effect it may have on the neutron absorption cross-section of the alloy.

An alloy according to the invention consists of zirconium, copper, aluminium and niobium, the proportions by Weight in the alloy being 0.253% Cu, 0.255% Al, 0.25-5% Nb, and the remainder zirconium, except for unavoidable impurities. I

The addition to alloys of zirconium and copper of aluminium, with the optional addition of niobium, has been found to improve the creep strength of the material. Aluminium raises the 01/ B transformation temperature of zirconium and strengthens the material, but this is associated with appreciable hardening and poor hot working characteristics. The eutectoid-type elements copper and niobium, on the other hand, lower the transformation temperature, and, if added in conjunction with aluminium, serve to ameliorate the adverse influence of this element on formability, and at the same time improve the mechanical properties from room temperatures up to 500 C.

Furthermore, both aluminium and niobium have low neutron absorption cross-sections and are thus permissible additions, up to several percent, from nuclear considerations. The absorption cross-section of copper is rather higher, but only relatively small percentages are required in order to produce corrosion-resistant alloys.

It is accordingly desirable to restrict the range of the additions of aluminium and niobium to a zirconium-copper alloy so that a preferred range of composition of alloys according to the invention is as follows: 0.5 %2% Cu, 13% Al, 1-3% Nb and the remainder Zr, all percentages being by weight.

Both copper and aluminium form insoluble intermetallic compounds and act mainly as precipitation hardeners. Niobium, on the other hand, does not form an intermetallic compound with zirconium and its solubility in a Zr changes appreciably with increasing temperature. Hence, the properties of zirconium alloys containing niobium may be improved by solution treating and ageing in the a. range.

To demonstrate the improvement in creep strength obtained by the addition of a combination of aluminium and niobium to a binary zirconium-copper alloy, creep tests were conducted on an alloy of zirconium with 1% of copper, and an alloy of zirconium with 1% of copper, 2% of aluminium and 2% of niobium. Test specimens of each alloy were stressed at 6000 pounds per square inch at 500 C. for a period of approximately 500 hours. The minimum rates of creep strain observed were as follows:

In./in./hour Zirconium, 1% copper alloy 6.2X10

Zirconium, 1% copper, 2% aluminium, 2% niobium alloy 1.0 10* The effect of the combined addition of aluminium and niobium is thus to reduce the minimum creep rate of the alloyed zirconium by a factor of approximately six.

The usual source of zirconium for use in the production of zirconium alloys is the commercial material known as sponge zirconium. Impurities may thus arise from the use of this material.

Arising from the ability of zirconium to oxidize and to absorb atmospheric and other impurities readily at elevated temperatures, the constituents of the alloy should be melted in a vacuum arc furnace. Apart from these restrictions the alloys of the invention can be readily fabricated by constructional methods.

Impurities in the base zirconium sponge should be kept to a minimum.

It is believed that quaternary Zr/Cu/Nb/Al alloys possess a combination of properties which make them ideally suited for reactor applications.

What we claim is:

1. An alloy consisting of zirconium, copper, aluminium and niobium, the proportions, by Weight, of the constituents in the alloy being 0.25-3% copper, 0.25-5% aluminium, 0.25-5% niobium, and the remainder zirconium, except for unavoidable impurities.

2. An alloy consisting of copper, aluminium, niobium and zirconium, the proportions, by weight, of the constituents in the alloy being 0.52% copper, 13% aluminium, 13% niobium, and the remainder zirconium, except for unavoidable impurities.

3. An alloy consisting by weight, of 1% copper, 2% aluminium, 2% niobium, and the remainder zirconium, except for unavoidable impurities.

References Eited in the file of this patent UNITED STATES PATENTS Marsh et al. Mar. 5, 1957 Raine et a1. Feb. 9, 1960 OTHER REFERENCES 

1. AN ALLOY CONSISTING OF ZIRCONIUM, COPPER, ALUMINUM AND NIOBIUM, THE PROPORTIONS, BY WEIGHT, OF THE CONSTITUENTS IN THE ALLOY BEING 0.25-3% COPPER, 0.25-5% ALUMINUM, 0.25-5% NIOBIUM, AND THE REMAINDER ZIRCONIUM, EXCEPT FOR UNAVOIDABLE IMPURITIES. 